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It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

I would have thought it's analogous to two volcanos going off, or hundreds. Nuclear winter happens as a result of full-scale nuclear war. A couple of nukes, I assume the cooling effect is negligible, at least on a global scale.

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

Given a certain set of criteria it probably does happen. are those criteria likely to ever manifest in reality? Probably not. Very short term cooling effects are likely & happen to some degree but long term nah.

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

We (humans) have not yet made a bomb that big. Even the Tsar Bomba isn't big enough for that.

We're talking energy release on the order of a 15 km diameter asteroid impact, such as is likely to have caused (or at least heavily contributed to) the Cretaceous Paleogene extinction event which killed off all the non-avian dinosaurs.

It would take a full scale international nuclear war to release that kind of energy, and I'm not sure it would be a long enough winter to compare with the one mentioned above.

Hiroshima and Nagisaki were altogether tiny explosions on the scale of nuclear weapons. We had much better technology in the following decades (see Tsar Bomba), and are many more decades past that, now.

IDK about the density of the cities as pertains to this discussion. The bombs themselves didn't create much burning. The widespread destruction of buildings and power lines is what created the ensuing firestorm. The Japanese didn't even believe that the bombs were a problem. They were convinced that we sent in massive waves of fire bombers behind the nukes, which of course we did not have the logistical capacity to have done.

I don't think any single nuclear explosion can trigger nuclear winter.

I don't think you can fit an actual nuclear weapon in a suitcase, either. You can fit a dirty bomb in a suitcase, but initiating a multi-stage nuclear event is not child's play.

I've picked out a few potential mountain tops from which to view and photograph the Supermoon this Sunday.

I see LOTS of articles posted saying "How to view the Supermoon this weekend", and they all say pretty much "look up"

The mountains I've picked do not all of 360 views of the horizon. Some of them are wooded and only have outlooks in one or some directions. And nothing I can find online is telling me from which direction, the moon will rise.

Well Timmy, the Earth spins around so that the Sun rises in the East and sets in the West.

So where do think the Moon rises and sets?

Well, considering it's a "supermoon", it's a full moon, which means it's the opposite side to the sun. As the sun sets, the moon rises. So you want an easterly view. We're approaching winter (or summer) solistice, so bear in mind the further north (or south) from the equator you are, the further south (or north) from true east the sun will set. Which means the moon will rise further north (or south) from true east.

I can't leave you guys alone for a few hours without this?
Who pooped in your cheerios, poopadoop?

Anyway.

The Earth spins from West to East, so celestial objects appear to rise from the East and set in the West.

I totally encourage anyone to get out and do some sky watching, but don't get too, too worked up about a perigee syzygy (super moon). It's basically just a full moon unless you're an avid moon watcher. You kinda want to compare it to a full moon from a month or two ago or a month or two from now to get a sense of how it was "super."

So plan on going back to said mountain in a month or two to really appreciate how it's not too much less super under any conditions.

What sort of preparation would one have to do to recreate a process like this, and is it because it's complicated that we don't see very many of them?

Hi, Panther.

The short answer is money. A cheap, used, low resolution microscope is going to cost in the tens of thousands. Those jobs can't go anywhere near the zoom shown in the video.

Something capable of producing that video definitely cost in the millions, perhaps up to 10 million... for the microscope. That's just one piece of the puzzle, and while that cost is a big chunk of the budget, it's not more than half the lifetime cost of building a lab around that machine, maintaining it and staffing it with high dollar technicians.

"That would be very exciting. It would mean you could generate something from nothing," he explained.

What absolute fucking twaddle.

You're not creating something from nothing... you're creating a stable atom from energy. So it's not like it was pulled out of a magic vacuum hat. If there's energy there, it's not a true vacuum. So he's talking out of his arse.

Rip space apart indeed. You might as well be worried about blinding god with that fucking giant laser pen.

I'm not sure what is lost in translation from the Chinese to English, but that's odd phrasing for pair production, which is all they seem to be talking about. However, pair production happens just over 1.022 MeV, which is way less than their current laser, so it's possible the article isn't talking about anything associated with this, even though it specifically mentions pair production.

My gut says this is just an intersection of QM definitions being used colloquially by the reporter.

There's literally not enough of a hint at actual physics to even guess at what the scientists are actually trying to accomplish.

Originally Posted by BananaStand

So is space an empty vacuum or not?? If it is, then how are there electrons in it? Wouldn't that make it not-empty??

Is the vacuum empty? No, not really.
We cannot measure absolute energy, only relative energy. We can measure if and how much one thing's energy is higher than another thing's energy, but we cannot really know if any of the values is exactly 0 J of energy. This is obvious to anyone who has studied electronics. You can't measure the potential (short for: electrical potential energy) at a point, you must measure the potential across 2 points. It is common to set ground to 0 V as a convenience, but it is not possible to prove that ground is 0 V. This is why we only really talk about the delta-V, or change in potential, but in short-hand, this specific use of language is thrown by the wayside.

The best way to describe the vacuum is not that it's "empty," but that it bears the minimum possible allowable energy in this universe (at this time).

One of the odd universe-ending physics things is spontaneous tunneling of the vacuum energy to a lower state. Meaning that the vacuum is tiny, but could be tinier if not for some unknown barrier keeping it from falling. However, QM says that all non-infinite barriers have a non-0 probability of not working (in a process called quantum tunneling). This means that IF the universe's current vacuum energy is not its absolute minimum, AND IF the barrier preventing the universe's vacuum energy from reaching its minimum energy is non-infinite, THEN at any given moment, the universe as we know it will cease to exist at some infinitessimal point. That ceasing of existence as we know it will expand out radially from that point at the speed of light, and will annihilate the universe with an event horizon that expands at the speed of light without anything to oppose it. Our universe would simply end in one instant without any warning that it was going to end.

You think a Fox News article with this as the leading headline is "physics"?

Dude.

More telling was that the article was originally reported in The Sun. Not a bastion of scientific literature.

Originally Posted by Savy

There is no such thing as a perfect vacuum. You are correct in what you say.

Even if there are no particles there energy is enough to make matter so yeah vacuums don't exist.

In general the energy is not enough to precipitate particles out of the "empty" space. However, in certain environments, like the extreme spacetime curvature near a black hole's event horizon, particle creation from the ambient, sloshing energy of that curvature can create particles, carrying away some of that energy.

Originally Posted by OngBonga

What absolute fucking twaddle.

You're not creating something from nothing... you're creating a stable atom from energy. So it's not like it was pulled out of a magic vacuum hat. If there's energy there, it's not a true vacuum. So he's talking out of his arse.

Rip space apart indeed. You might as well be worried about blinding god with that fucking giant laser pen.

Despite the tone, this seems all good.

We can dicker over the phrase "true vacuum" and whether that should be a theoretical ideal or if that should be an observed quantity, but we're not really disagreeing on what anything is or means, just which collection of syllables means what.

Nothing in the article said it was going to create stable atoms. It'd be far, far more likely that it created random ions and isotopes, the vast majority will decay in nuclear reactions over time. Most of those decays will happen basically immediately, but plenty will have longer half-life times and will linger about slowly irradiating their surroundings.

I mean, taking energy and mashing it all together to make, well either an ion or even a stable atom, that's kind of like collecting all the ash up and making a log out of it. Is this actually possible, in theory? I was always told it wasn't possible, but I was never really convinced. I mean it's impossible because we're going to lose ash, which means we never reproduce exactly what was burned, but let's assume we can collect 100% of the ash and heat... I'm not asking if we can do it, because obviously we can't, but could a super-intelligent alien?

I mean, taking energy and mashing it all together to make, well either an ion or even a stable atom, that's kind of like collecting all the ash up and making a log out of it. Is this actually possible, in theory? I was always told it wasn't possible, but I was never really convinced. I mean it's impossible because we're going to lose ash, which means we never reproduce exactly what was burned, but let's assume we can collect 100% of the ash and heat... I'm not asking if we can do it, because obviously we can't, but could a super-intelligent alien?

In theory, the log can be unburned at a cost of even more entropy than it originally produced in burning, plus all the released energy in the form of heat and light and noise, etc. and you have to have all the original matter to reassemble into the pre-combusted molecules.

Maybe a talented chemist could coax that chemical process to run in reverse on the microscopic level, but IDK.
I suspect that un-rusting iron would be easier to do. Rust and fire are chemically similar in that they're both exothermic oxidation processes.

Quantum Mechanically, all processes are reversible, so entropy doesn't exist at the particle level. Entropy is an emergent property of many-particle systems. "Many" is still poorly defined and understood. Active research in this field is called mesoscale (in-between-sized) physics.

In theory, everything we see happening "forward in time" can be seen "backward in time," but dissipating energy in the form of heat, light, sound, etc. has an incredibly low probability of happening in reverse. A thing exploding from a central source is easy to create. Unexploding a thing by trying to get all the particles moving exactly just so to come together in a dynamite stick is, like, super hard [citation needed].

***
Upon further reflection, all those unstable isotopes will decay into stable atoms or ions, so the assertion that it creates stable atoms isn't really that terrible.

So this "incredibly low probability"... am I to assume that while I sit by my fire, the vast majority of the heat is moving from a warm place to a cool place, but not quite all of it? I assume the heat moving from the cooler area to the warmer area is utterly overwhelmed by the heat moving in the opposite direction, rather like trying to piss up a waterfall.

What causes that "incredibly low probabiltiy"? Is it environmental, or completely random? Or do we not know?

So this "incredibly low probability"... am I to assume that while I sit by my fire, the vast majority of the heat is moving from a warm place to a cool place, but not quite all of it? I assume the heat moving from the cooler area to the warmer area is utterly overwhelmed by the heat moving in the opposite direction, rather like trying to piss up a waterfall.

What causes that "incredibly low probabiltiy"? Is it environmental, or completely random? Or do we not know?

Heat transport is a function of relative temperature between 2 sources, and some coefficient of heat diffusion, which characterizes the specific material through which the heat is flowing.

Newton's Law of Cooling:
dT/dt = -k(T(t) - T_inf)

The time rate of change of the temperature of an object is proportional to the difference in temperature between that body and its "ambient" environment.

Solving this 1st order differential equation yields an equation of the form
T(t) = T_inf + (T_init - T_inf)*e^(-kt)

That e^(-kt) part is what we're looking at now.

I'm running out of time.
I'm trying to get to a point where I state that the transport of heat from low temp to high temp is exponentially less than the transport from high temp to low temp the further apart the high and low temps are. When the high and low temps approach the same value due to this exchange process, the amount of transport approaches equilibrium. I.e. when things are the same temperature, the amount of heat flowing between them is the same.

(I need to add a lot of caveats to pin that down to a specific case, but I hope you can do some of that yourself. Feel free to ask for clarity where I trailed off. I'll come back to it later.)

I'm trying to get to a point where I state that the transport of heat from low temp to high temp is exponentially less than the transport from high temp to low temp the further apart the high and low temps are. When the high and low temps approach the same value due to this exchange process, the amount of transport approaches equilibrium. I.e. when things are the same temperature, the amount of heat flowing between them is the same.

You might be surprised how logical this is. Two objects of the same temperature are still exchanging heat, it's just the heat transfer cancels out to net neutral. Something that is hot is losing more heat than it gains, and the hotter it is the faster it loses its heat relative to the cool object.

Is this a function of radiation? Is it simply the case that the hot object radiates more than the cool object?